Converting a PDF to grayscale removes all color information from every page, replacing it with shades of gray from pure black to pure white. The two practical reasons to do this: preparing a document for black-and-white printing to reduce color ink or toner costs, and reducing file size when embedded color photos are larger than necessary.
How grayscale conversion affects content
Text that was black stays black — there is no visible change. Text that was colored (red headings, blue links) becomes dark gray; the lightness of the gray corresponds to the perceived luminance of the original color.
Images are converted using luminance weighting: a bright yellow converts to a light gray; a dark blue converts to a dark gray. The result approximates what the image would look like printed on a black-and-white laser printer.
Charts and graphs with color-coded data require attention: if two data series were differentiated only by color (one red line, one blue line), they may become indistinguishable when converted to gray. Review any charts in the output to confirm the data remains legible.
File size reduction
Color images in PDFs are typically stored as RGB (three bytes per pixel). A grayscale image requires one byte per pixel — a 3× reduction in the raw image data. Whether this translates to a proportional file size reduction depends on how the PDF is structured: JPEGs embedded in the PDF are already compressed, so the size reduction comes from re-compressing the image as grayscale.
For text-heavy PDFs with few images, grayscale conversion produces little file size reduction because text is stored as vector paths, not pixels. For image-heavy PDFs (catalogs, reports with photographs), the reduction can be significant.
Grayscale vs black-and-white
Grayscale and pure black-and-white (bitmap/1-bit) are different. Grayscale retains 256 shades of gray — photographs look like traditional black-and-white photos. Pure black-and-white converts each pixel to either fully black or fully white (via a threshold or dithering algorithm) — the result looks like a fax or a photocopy.
Filum's Grayscale PDF tool produces true grayscale output (256 shades), not bitmap. This preserves photograph detail and is the correct choice for most documents.